Add helper method to query the latency on the Queue object

app/models/solid_queue/queue.rb

@@ -40,6 +40,19 @@ def size
       @size ||= ReadyExecution.queued_as(name).count
     end
 
+    def latency
+      @latency ||= begin
+        now = Time.current
+        oldest_enqueued_at = ReadyExecution.queued_as(name).minimum(:created_at) || now
+
+        (now - oldest_enqueued_at).to_i
+      end
+    end
+
+    def human_latency
+      ActiveSupport::Duration.build(latency).inspect
+    end
+
     def ==(queue)
       name == queue.name
     end

test/unit/queue_test.rb

@@ -2,6 +2,8 @@
 
 class QueueTest < ActiveSupport::TestCase
   setup do
+    freeze_time
+
     5.times do
       AddToBufferJob.perform_later "hey!"
     end
@@ -39,4 +41,35 @@ class QueueTest < ActiveSupport::TestCase
       @default_queue.resume
     end
   end
+
+  test "return latency in seconds on each queue" do
+    travel_to 5.minutes.from_now
+
+    assert_in_delta 5.minutes.to_i, @background_queue.latency, 1.second.to_i
+    assert_equal 0, @default_queue.latency
+
+    @background_queue = SolidQueue::Queue.find_by_name("background")
+    @default_queue = SolidQueue::Queue.find_by_name("default")
+    travel_to 10.minutes.from_now
+
+    assert_in_delta 15.minutes.to_i, @background_queue.latency, 1.second.to_i
+    assert_equal 0, @default_queue.latency
+  end
+
+  test "returns memoized latency after the first call" do
+    travel_to 5.minutes.from_now
+
+    assert_in_delta 5.minutes.to_i, @background_queue.latency, 1.second.to_i
+
+    travel_to 10.minutes.from_now
+
+    assert_in_delta 5.minutes.to_i, @background_queue.latency, 1.second.to_i
+  end
+
+  test "return human latency on each queue" do
+    travel_to 5.minutes.from_now
+
+    assert_match (/5 minutes/), @background_queue.human_latency
+    assert_match (/0 seconds/), @default_queue.human_latency
+  end
 end